The rapid loss of TL (Thymus-Leukemia) antigenicity from the surfaces of mouse thymocytes and leukemia cells during incubation with TL alloantiserum in vivo and in vitro at physiological temperatures, as measured by cytotoxicity using lytic complement, has been termed antigenic modulation. Preliminary in vitro experiments indicate that modulation does not involve a disappearance of TL antigens from the cell surface, but rather an alteration in the organization of antigens within the cell surface following alloantibody attachment. We will attempt to determine the specific cell surface alterations which occur during modulation in vitro. Changes in the topography and topology of TL antigens on the cell surface during modulation will be analyzed by immunofluorescence, immuno-electron microscopy, and cytotoxicity, using labeled bivalent and univalent TL alloantibody in combination with various methods of stimulating and inhibiting modulation. The amount of TL antigen remaining on the cell surface, exfoliated from the cell surface, and internalized by pinocytosis during modulation will be quantitated using I125-labeled alloantibody and analysis by gamma ray spectrometry, electron microscope autoradiography, and radioimmunoprecipitation. Lactoperoxidase radioiodination of TL antigens on the cell surface and analysis by radioimmunoprecipitation will be used to identify gross conformational changes in the disposition of TL antigen within the cell surface membrane. Radioisotope-labeled complement components will be used to determine which steps in complement-mediated cell lysis are affected by modulation.